Conventional contact plugs are made of CVD (chemical vapor deposition) tungsten (W) for making contact to the front end devices (FEOL). The CVD (chemical vapor deposition) W process is a conformal process which tends to result in center voids and seams within the plug. Due to the shrinking contact feature sizes and the increase in contact aspect ratio, it becomes more and more challenging to fill the structure by a CVD W process and the increased parasitic resistance. Furthermore, the shrinking contact feature sizes result in a dramatic increase of parasitic resistance from the overall CVD W plug structure due to the high resistivity of CVD W and the highly resistive Ti/TiN liner stack. The poor filling of the structure by CVD W process can significantly degrade the performance of the whole chip due to the increased contact resistance with the voids and seams.
Electroplating has attracted great interest in VLSI fabrication since the discovery of copper damascene process with superfilling capability into submicron sized features. For example, see D. Edelstein, et al., IEEE 1997 Intl. Electron Devices Meeting Digest, 773 (1997); P. C. Andricacos, et al., IBM J. Res. Develop., 42, 567 (1998) and U.S. Pat. No. 6,709,562 B1, disclosures of which are incorporated herein by reference. There have been a lot of studies to search for superfilling systems of other materials by electrodeposition. However, only Ag and Au have been reported to have some superfilling capabilities in addition to Cu. Along these lines see T. P. Moffat, et al., Electrochem. Solid-State Lett. 4(4), C26 (2001), T. P. Moffat, et al., J. Electrochem. Soc., 149, C432 (2002) and D. Josell, et al., Electrochem. Solid-State Lett., 8(3), C54 (2005).
Rhodium (Rh) is one of the platinum group metals. It is a noble and precious metal with excellent corrosion resistance. Electroplated Rh has been used for jewelry applications for its highly reflective appearance. It has been used to make electrical contacts due to its low and reliable contact resistance. Rh is also a well-known catalyst for gas conversion. See D. Pletcher, et al., J. Electroanalytical Chem., 421, p 137 (1997), D. Pletcher, et al., J. Electroanalytical Chem., 421, p 145 (1997) and R. T. S. Oliveira, et al., J. Electroanalytical Chem., 569, p 233 (2004).
Rhodium has negligible diffusion rate in Si, (C. S. Peterson, et al., J. Appl. Phys., 53(7), p 4866 (1982)) which gives great advantage over Cu as an alternative metal for CVD tungsten (W) as contact plugs in VLSI (A. Topol, et al., VLSI conference proceeding (2005) The resistivity of the current CVD W is reported to be approximately 20 micro-ohm-cm within the contact plugs. Filling of contact vias of 32 nm node and beyond becomes a great challenge for CVD tungsten process due to void formation during the conformal filling, as mentioned above. Also, see FIG. 2.